(1) Field of the Invention
The present invention relates to an ultraviolet ray-transparent optical glass material comprising a transparent silica glass and usable for lithography, optical CVD and optical cleaning using an irradiation source for ultraviolet rays in a wavelength range of about 160 to 300 nm, and a method of producing the same. More particularly, the present invention relates to an ultraviolet ray-transparent optical glass material comprising a transparent silica glass usable for lenses, photomasks for super LSI, prisms, window materials and lamp materials which must have a high transmittance for radiations from excimer lasers and excimer lamps in the vacuum ultraviolet ray band and the ultraviolet ray region and, especially, a transparent silica glass useful for F2 excimer laser beam having a wavelength of 157 nm, and a method of producing the same.
(2) Description of the Prior Art
Conventional silica glasses have certain advantageous properties and certain disadvantageous properties and thus are used after consideration of the purpose. For example, for the optical materials for KrF laser beam having a wavelength of 248 nm or ArF excimer laser beam having a wavelength of 193 nm, usually a high purity synthetic silica glass having a significantly high content of OH groups or treated by a hydrogen treatment is employed, as disclosed in Japanese Unexamined Patent Publication No. 4-97922. In this type of synthetic silica glass, the resistance of the glass to a laser beam is enhanced by increasing the content of SiOH groups to make the glass network structure soft. Namely, to prevent generation of defects due to formation of precursory defective structures and a breakage of strained Sixe2x80x94Oxe2x80x94Si bonds, the number of the strained bonds must be made small. Thus, in the above-mentioned silica glass, to decrease the strained bond content, the strain of the skeleton structure of SiO2 should be decreased to prevent the formation of the defective structures, by increasing the content of Sixe2x80x94OH groups. In the prior art, the glass for ArF excimer laser beam was produced while preventing the formation of the above-mentioned unstable molecules, and was employed in practice. However, it is known that the transmitting property of the silica glass for vacuum ultraviolet rays is increased with a decrease in the content of OH groups in the silica glass and, when the glass is used for the vacuum ultraviolet rays, which will be referred to as VUV hereinafter, having a relatively short wavelength, the light absorption of the glass due to the OH groups hinders the transmission of the above-mentioned ultraviolet rays through the glass, and as a result, the silica glass having a high content of the OH group is not suitable as an optical material for an Xe2*excimer lamp beam having a wavelength of 172 nm and an F2 excimer laser beam having a wavelength of 157 nm. In a preferable method other than the above-mentioned method in which the OH group content is made high to prevent the breakage of the skeleton structures, the silica glass is subjected to a hydrogen treatment to cause hydrogen to be dissolved in the silica glass. This method is considered preferable.
However, this method exhibits several problems. Namely, the resultant silica glass has a disadvantage in its low durability after releasing hydrogen (H2). Also, since the hydrogen reacts with the skeleton structure of the silica glass to producexe2x89xa1Sixe2x80x94H groups and HOxe2x80x94Sixe2x89xa1groups, when the silica glass is used over a long period, the durability of the glass is significantly decreased. Further, when the silica glass is used as a tube material for a lamp, hydrogen H2 and water H2O are discharged from the glass into the tube, to cause defective structures to be formed in the silica glass. Therefore, the light transmittance of the glass decreases and the light intensity of the lamp decreases.
A still another method, in which a silica glass is doped with fluorine to provide an optical silica glass material for VUV, has been proposed. When fluorine is doped in a sufficient large amount in the silica glass, not only does the resultant doped glass exhibits an enhanced resistance to ultraviolet rays, similar to that of the silica glass containing a large amount of OH groups, but it also exhibits no shift of the absorption end of vacuum ultraviolet ray transmittance, due to the OH groups, to a longer wavelength side, and thus a silica glass material having excellent vacuum ultraviolet ray-transmission property can be obtained. To impart a sufficient high resistance to excimer laser beam to the glass by the above-mentioned method, the glass must be doped with fluorine in an amount of several hundreds ppm or more. However, since the doped fluorine causes the refractive index of the silica glass to be reduced, a distribution of the refractive index occurs and strial are generated.
An optical ununiformity such as strial cannot be removed by a heat treatment or a melt-drawing or expansion. Also, when the fluorine-doped silica glass is heat-treated, the resultant silica glass exhibits absorption bands at 7.6 eV and 5.0 eV. For example, if the fluorine-doped glass is used as an optical material for an excimer laser beam, a portion of the doped fluorine is released from the glass due to the irradiation of the laser beam, and is emitted as F2 which has a high etching property, from the surface of the glass, and the optical device is affected by the emitted fluorine. For the above-mentioned reasons, the fluorine-doped silica glass is not suitable for a vacuum ultraviolet ray-transparent optical material such as a stepper projection lens which has a large aperture and must have a high uniformity.
Therefore, when the content of the OH group in the silica glass is reduced, although the transmission, in the resultant glass, of the vacuum ultraviolet rays is enhanced, the transmittance of the glass to the vacuum ultraviolet rays significantly falls with a lapse of ultraviolet ray-irradiation time. As mentioned above, when the transmittance falls with the irradiation time, not only the absolute value of the reduced transmittance but, also, the stability in the ultraviolet ray transmittance of the glass must be considered. For example, when the silica glass is used as a window material or a collective lens for a light CVD using a excimer lamp with a wavelength of 172 nm and for ultraviolet ray dry-cleaning, and ultraviolet rays, particularly VUV rays with a wavelength of less than 200 nm, are continuously irradiated to the glass for a long period, defects are usually gradually generated in the glass, and the ultraviolet ray transmittance of the glass falls in proportion to the irradiation time.
In view of the above-mentioned phenomena, a synthetic silica glass which has a low OH group content, contains no ultraviolet ray-absorbing defective structures and no precursory structures of the defective structures, and exhibits a high resistance to generation of defective structures and precursory defective structures even when the glass is continuously exposed to a heat treatment or a VUV ray-irradiation for a long period, is considered as a glass exhibiting an excellent ultraviolet ray-transmittance. However, an ultraviolet ray-transparent optical glass material comprising the above-mentioned type of synthetic silica glass has not yet been provided in practice.
An object of the present invention is to provide an ultraviolet ray-transmissible optical glass material comprising a synthetic silica glass which has a high transmittance of ultraviolet rays, particularly at a wavelength of 157 nm which is a lasing wavelength of F2 excimer laser, which glass is an optically stable substance free from change in composition and in light transmission, and which glass does not exhibit an increased light absorption or a reduced uniformity even after a heat treatment at a high temperature or VUV ray irradiation is continuously applied to the glass over a long time, and a method of producing the ultraviolet ray-transparent optical glass material.
The above-mentioned object can be attained by the ultraviolet ray-transparent optical glass material and the method of producing the same, of the present invention.
The ultraviolet ray-transparent optical glass material of the present invention comprises a synthetic silica glass produced from a silicon compound having a high degree of purity by a soot method in which the silicon compound is flame-hydrolyzed, the resultant soots are accumulated to form a porous silica preform body and the porous silica preform body is converted to a transparent glass by heating, containing substantially no impurity other than hydroxyl group, and having a difference between highest and lowest fictive temperatures of the glass of 50xc2x0 C. or less, and a transmittance of ultraviolet rays at a wavelength of 157 nm through an optical path having a length of 10 mm of 60% or more.
Also, the method of the present invention for producing the ultraviolet ray-transparent optical glass material defined above comprises firstly heat-treating a porous silica preform body produced in the soot method, at a temperature which does not cause the porous silica preform body to be transparent; and secondly heat-treating the first heat-treated porous silica preform body at a temperature which is higher than the first heat-treatment temperature and causes the first heat-treated porous silica preform body to be transparent.